How does water flow on flat land

The water cycle

The water on earth is always on the move. Huge amounts of it are constantly moving - between sea, air and land - in an eternal cycle in which not a single drop is lost.

The engine of the water cycle is the sun: It heats the water of the seas, lakes and rivers so much that it evaporates. Plants also release water vapor into the atmosphere through tiny openings. The humid air rises, tiny water droplets gather in the air and form clouds. As rain, hail or snow, the water falls back into the sea or onto the earth. If it falls on the ground, it seeps into the ground, supplies plants or flows through the ground, over streams and rivers back into the sea. The eternal cycle of evaporation, precipitation and runoff starts all over again.

The water cycle has been around for almost as long as the earth has existed. He ensures that living beings on our planet are supplied with fresh water. And not only that: Without the water cycle, the weather as we know it would not exist.

What water can do

No matter whether we drink tap water, jump into a lake or are surprised by a downpour - we are constantly in contact with water. And not only that: we are made up of water ourselves, around two thirds of which is actually water. Without question, water is part of our everyday life. But what seems completely normal to us has all kinds of peculiarities. And the water owes this primarily to its structure.

Everything that exists on this earth is made up of tiny building blocks, the atoms. This is also the case with pure water: It is a combination of two hydrogen atoms and one oxygen atom. These combine to form a water molecule, H for short2O. The individual water molecules are only loosely connected to one another.

This loose cohesion ensures that the connection between the molecules breaks at high temperatures: the water evaporates. If, on the other hand, it cools down significantly, the molecules organize themselves into a solid, regular lattice, the ice. The special thing about it: In its solid form, water has a larger volume than in its liquid state.

The arrangement of the water molecules also ensures another property: the surface tension of the water. Because of this tension, water spiders and water striders can easily walk on a pond. But water can do even more: it is able to dissolve substances. Small grains of salt or sugar dissolve completely in water. Sea water, for example, contains large amounts of salt that we can taste but not see.

We owe the fact that lemons ripen on the island of Mainau on Lake Constance to another ability of water: it can store heat. Lakes or seas heat up in summer and keep the heat for a long time. That is why the temperatures on the coast fluctuate less than inland. Far from the coast, the temperature differences between day and night and between summer and winter are much greater than near the sea.

The blue planet

Seen from space, the globe appears in a strong blue. This is because almost three quarters of the earth is covered with water. Small amounts of water are transparent, but from a certain depth onwards it becomes more and more blue. Because we see the mighty oceans blue, the earth is also called "the blue planet". The term south of the equator is particularly applicable. Because the southern hemisphere is almost completely covered by the sea, because a large part of the continents have migrated to the north due to plate movement.

The vast oceans contain almost all of the water on earth. There is a lot of salt dissolved in sea water, which is why it is not suitable as drinking water. The little fresh water on earth is frozen mainly in glaciers and ice caps. Only a tiny fraction of freshwater is found in groundwater, in lakes and rivers, or in the air.

But the view from the outside is deceptive: the earth's surface is largely covered by water, but measured by the diameter of the earth, the oceans are only a very thin layer. Therefore, the water makes up only a fraction of the earth's mass. For comparison: if the earth were the size of a basketball, all the water on earth would fit into a table tennis ball. And the drinking water would be proportionally even smaller than a single popcorn.

Quick-change artist water

It is well known that water is liquid. However, this is not always the case. In nature, water occurs in three states: as liquid water, as gaseous water vapor or as solid ice. Depending on the external conditions, it passes from one state to the other.

The state of the water depends on the pressure and its temperature. If liquid water exceeds the boiling point, it evaporates and floats in the air as gaseous water vapor. Even when evaporating at room temperature, water changes into a gaseous state. However, this happens more slowly than when evaporating. If, on the other hand, the temperature drops below 0 ° Celsius, the water freezes to ice. As soon as water changes its state between liquid, gaseous or frozen, it changes its properties.

The special thing about water is that it has its greatest density at 4 ° Celsius and takes up little space. When it freezes into solid ice, it expands and increases in volume. At the same time, its density decreases. That is why ice is lighter than water with the same volume. That is why icebergs can drift in the sea. For the same reason, a lake freezes over from above and not from below in winter. That's a good thing, because otherwise we wouldn't be able to skate until the lake was completely frozen from the bottom to the surface.

So water expands when it freezes. If you prevent it from doing so, there is tremendous pressure on it. Anyone who has ever forgotten a bottle of water in the freezing cold outside knows the consequences: after a while the bottle bursts and the ice oozes out. In this way, ice can also break stone. This happens when water flows into cracks in the rock, freezes there and pushes outwards due to the expansion. If pieces of stone flake off as a result of this force, it is called frost splitting. Anyone who has ever driven into a pothole knows the consequences. Here the constant alternation of wetness and frost have really damaged the asphalt.

How do clouds form?

How clouds are formed can be observed particularly well on cold winter days: when you breathe out, it steams from your mouth - a whitish veil hangs in the air. It forms when the moist, warm air you breathe meets colder air. Because warm air can store a lot of moisture - significantly more than cold air. If the warm air cools down, it can no longer absorb as much water. Then the excess water collects into small water droplets that float in the air and become visible as a white veil. It is very similar with the "real" clouds.

The power of the sun heats the land and the water on the surface. The heat turns part of the liquid water into gaseous water: it evaporates. Because warm air is lighter than cold air, it rises. If the warm, humid air cools further upwards, the excess water collects as droplets around tiny dust or soot particles. It is also said that the water condenses. The drops are still so small and light that they float in the air. A cloud has arisen.

So clouds always form when warm air cools down. This can happen when the ground and the air above it warms up and rises. Even if the wind drives the air up a mountain, warmer air is forced upwards. At altitude it cools down, clouds form. The same thing happens when a zone of warm air meets a zone of cold air. The cold air lets the lighter warm air rise and clouds form again!

But it doesn't rain immediately from every cloud. Only when the water droplets combine to form larger drops due to the movement of air and are heavy enough, they fall back on the earth as rain. If the temperature is below 0 ° Celsius, the drops freeze into ice crystals. Then the precipitation falls as snow, in thunderclouds also as small sleet or as large hailstones.

There are also clouds that form just above the surface of the earth. This often happens in autumn when the air continues to cool. The whole landscape then appears blurred whitish. If you can see less than a kilometer through this white haze, it is called fog.


It doesn't matter whether it's raining, hailing or snowing - the clouds are “to blame” for this. Because without clouds there would be no precipitation. However, it depends above all on the temperature, whether there is a downpour or a wild snowstorm.

Most of the precipitation on earth falls as rain. When small water droplets collide in a cloud, they combine to form larger and heavier droplets. Are they too heavy to float; if the temperature is above 0 ° Celsius, they fall on the earth as rain.

When the air temperature is very low, precipitation no longer falls as rain, but as snow. The snowflakes grow from hexagonal ice crystals that stick together in very cold clouds with water droplets. If the ice formations are big and heavy enough, they dance down from the sky like snowflakes.

If, on the other hand, strong updrafts pull through a high cloud, there can be hail. Small drops from the lower part of the cloud are swirled upwards, where it is colder than below. There they freeze to form small ice balls, about the size of the heads of a pin. These ice balls are called sleet. If in a very high thundercloud with a strong wind the globules in the cloud are flung up and down several times, more and more raindrops freeze onto the globules. The more the ice balls are driven around in the cloud, the bigger and harder they become. From half a centimeter in diameter, these ice balls are called hail. Hailstones can grow larger than tennis balls and have often already done a lot of damage.

In contrast to precipitation that falls from clouds, there is also precipitation that occurs close to the surface of the earth. If the temperature on the ground drops overnight, the air can absorb less moisture. The excess water then settles on the ground, on plants or on objects: the moisture is clearly visible as dew. If the temperature falls below 0 ° Celsius at night, the water freezes on the objects and forms a whitish layer. Then one no longer speaks of dew, but of frost.

How is groundwater created?

Like water in a sponge, groundwater collects in small and large cavities under the earth. It occurs when rain or meltwater seeps into the ground or when water from streams, rivers or lakes flows through crevices into the subsoil.

Depending on whether the soil consists of loose sand or dense soil, the water descends faster or slower. And only when the downward flowing water hits a water-impermeable layer of rock such as clay, the seepage is stopped. Then the groundwater collects in the cavities of the ground above the impermeable layer and is stored therein. If the layer of “watertight” rock slopes down, the groundwater flows even down the slope towards nearby streams and rivers. The places where the groundwater emerges from the surface are called springs.

When it rains very heavily or a large amount of snow melts, more water collects on the earth's surface in a short time than can seep away. Then the water backs up. If it cannot drain away quickly enough, it will cause a flood.

As the groundwater flows through the various layers of the earth, it is filtered and purified. This is why drinking water can be obtained particularly well from groundwater.

The groundwater is part of the water cycle. It can stay inside the earth for less than a year or several million years. Under the Sahara, for example, researchers have discovered groundwater that has been under the desert sand for many thousands of years.

From trickle to stream - flowing waters

Gushing groundwater emerges from a spring and flows down the slope as a thin trickle or as a small stream: a flowing body of water has emerged. All rivers start out small. On their course towards the estuary they unite with other rivers and continue to grow until they become a river or even a broad stream. At its lower end, the flowing water flows into another river, into a lake or into the sea.

Streams, rivers or streams - names that come from our lips are precisely distinguished from one another by scientists (geographers). They can be divided according to their amount of water, their length or their width: If the flowing water is less than half a meter wide, it is called a trickle, if it is more than 2 meters wide, it is a stream. If the water swells up to 10 meters wide, it is a river. And if it gets even wider, the river can be called a stream. For example, one speaks of a river near the Amazon or the Nile, but the Rhine and Danube are also rivers.

The amount of water in the running water increases from the source to the mouth. Still, it flows slower and slower downwards. This is because the slope it flows down is steeper at the top than at the bottom. And because the water flows faster at the top and slower down the valley, it can drag more sand and debris along the upper course than at the lower. More sand and rubble is removed from the upper reaches of a river and more is deposited on the lower reaches.

Delta - watercourse between the river and the sea

The Nile pours mightily and sluggishly into the Mediterranean. Like all large rivers that flow through a plain, the African current slows down towards the mouth. From the slow current, the cargo of excavated rubble and sand sinks to the ground and is deposited. With these deposits the river builds its own obstacle that it has to flow around. The result is finely branched branches of sandbanks, scree slopes and river arms, which become wider and wider towards the mouth. From the air, this widely branched network looks like a triangle. Because of its shape, it is called delta, after the Greek letter of the same name.

Over time, the river accumulates more and more deposits, layer by layer. The course of the river spills its own mouth and the delta protrudes further and further into the sea: the river is lengthening. This process is particularly visible on the Nile. Its delta begins near Cairo and is now 160 kilometers long and 240 kilometers wide on the coast. And the Nile Delta is getting bigger and bigger: Shaped like a fan, it is constantly growing further into the Mediterranean.

In order for a delta to arise, further conditions must be met. The coast must be flat, the tides and ocean currents low, because only then will the sediments not be immediately removed by the moving ocean water. The right conditions exist, for example, on the lower reaches of the Po or Danube rivers. Both streams flow into the shallow sea in a delta.